<p></p>
<p>OK: With the additional memory barrier in place, the ROM grows by 150 bytes. The symbols <code>_xtimer_now64</code> and <code>_periph_timer_callback</code> have very different (and longer) assembly code. (<code>static inline _xtimer_now64()</code> is btw. not inlined; which really is a good thing on AVR. It is huge there.)</p>
<p>I still think that the added memory barrier is not really needed and just hides another bug. Let me do some more digging.</p>

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